1. Field of the Invention
This patent application is related to a method for obtaining scale factor calibration of a fiber optic sensor head.
2. Description of the Related Art
Fiber optic sensors have been configured as passive devices with high sensitivity and excellent reliability in high temperature and pressure environments. For some sensors, a change or drift in scale factor of the transducer element of the fiber optic sensor can occur, and this becomes a problem if scale factor drift appears between measurements, causing errors in the measurements. A change in scale factor can occur due to changes in material properties used in the construction of the sensor element, in particular for a sensor element exposed to and used in high temperature and/or pressure environments. Therefore, it is usually desired, if possible, to perform a pre-calibration of a sensor prior to a measurement in order to determine the absolute value of the signal amplitude and phase in a particular measurement. In permanent installations of fiber optic sensors, for example downhole in oil and gas wells, the sensors cannot be retrieved and recalibrated after being installed.
The objective of this invention is to provide a method for remote calibration of the scale factor of fiber optic electromagnetic sensors, without the need for retrieving the sensors to do recalibration. In particular it is an objective to be able to remotely calibrate the scale factor of permanently installed downhole fiber optic electromagnetic sensors, e.g. fiber-optic magnetic and electric field sensors, from the surface in seismic or downhole surveying. The signal in the optical fiber in combination with a known electrical calibration signal are used to perform a calibration of the scale factor of all the individual fiber optic sensors which are forming an array. This is obtained by using a method for doing remote scale factor calibration of fiber optic electromagnetic sensors, conversion unit and system.